Self-referencing and self-calibrating interference pattern overlay measurement

ABSTRACT

Two pairs of alignment targets (one aligned, one misaligned by a bias distance) are formed on different masks to produce a first pair of conjugated interference patterns. Other pairs of alignment targets are also formed on the masks to produce a second pair of conjugated interference patterns that are inverted the first. Misalignment of the dark and light regions of the first interference patterns and the second interference patterns in both pairs of conjugated interference patterns is determined when patterns formed using the masks are overlaid. A magnification factor (of the interference pattern misalignment to the target misalignment) is calculated as a ratio of the difference of misalignment of the relatively dark and relatively light regions in the pairs of interference patterns, over twice the bias distance. The interference pattern misalignment is divided by the magnification factor to produce a self-referenced and self-calibrated target misalignment amount, which is then output.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit under 35 U.S.C. § 120 as acontinuation of presently pending U.S. patent application Ser. No.15/869,150 filed on Jan. 12, 2018, the entire teachings of which areincorporated herein by reference.

BACKGROUND Field of the Invention

The present disclosure relates to the alignment of masks used inintegrated circuit (IC) manufacturing, and more specifically, toself-referencing and self-calibrating interference pattern overlaymeasurement methods and systems.

Description of Related Art

Fabrication of integrated circuits generally involves the formation ofmultiple integrated circuit patterns on one or more layers over asubstrate wafer. These patterns generally include numerous regionsformed through photolithography. Photolithography uses patterns todefine regions on a substrate. More specifically, with photolithography,a photoresist layer is formed on a substrate, and is exposed toradiation, such as ultraviolet light (UV), which is transmitted throughtransparent areas of a mask to cause a chemical reaction incorresponding regions of the photoresist. The photoresist is thendeveloped to produce a pattern of open areas that expose the underlyingmaterial, with the other areas of the material are still protected bythe photoresist. Depending on whether a positive or negative tone resistis used, the exposed or unexposed portions of the photoresist layer areremoved. The portions of the substrate not protected by the photoresistare then etched to form the features in the substrate.

The relative positioning and alignment, or “overlay,” between maskscontrols whether the resultant integrated circuits are formed properly.Minimizing overlay error is a significant concern in the manufacturingof integrated circuits. Overlay metrology minimizes overlay errors byusing overlay marks in the same layer as the functional circuitstructure. The overlay marks may include different patterns that maythen be scanned and/or imaged by an overlay metrology tool. Some overlaymarks (Moiré targets) combine to generate a diffraction pattern (Moirépattern) that can be measured to determine the accuracy of the overlayof the different masks. Many different types of overlay metrologypatterns have been developed to improve the accuracy of overlaymetrology measurements.

Advancing technology continues to make smaller structures in integratedcircuit (IC) devices. The complexity of advancing technology process hasput a heavy burden on lithography control parameters such as overlay formultiple layers. Having an overlay out of specification may result inopen circuits or shorts in the structures, which not only impactswafer/die yield, but also impacts process throughput due to thenecessity to rework the device.

SUMMARY

Various method herein establish a first Moiré target having a firstpitch on a first optical mask, and establish a second Moiré targethaving a second pitch on the first optical mask. The second Moiré targetis adjacent to, and aligned with, the first Moiré target. These methodssimilarly establish a third Moiré target having the second pitch on asecond optical mask, and establish a fourth Moiré target having thefirst pitch on the second optical mask. The third Moiré target isadjacent to the fourth Moiré target. The third Moiré target ismisaligned with the fourth Moiré target by a bias distance.

The first Moiré target and the third Moiré target form firstinterference patterns. The second Moiré target and the fourth Moirétarget also form second interference patterns. Further, the firstinterference patterns and the second interference patterns form a firstpair of conjugated interference patterns. Also, the first interferencepatterns and the second interference patterns form a first pair ofconjugated interference patterns.

The first Moiré target, the second Moiré target, the third Moiré target,and the fourth Moiré target are a first set of targets. Such methodsfurther establish a second set of targets that is identical to the firstset of targets, and is inverted relative to the first set of targets, onthe first optical mask and the second optical mask to produce a secondpair of conjugated interference patterns that is inverted relative tothe first pair of conjugated interference patterns.

Following this, these methods perform a first exposure using the firstoptical mask to produce an integrated circuit layer having the firstMoiré target and the second Moiré target of the identical, inverted,sets of targets. Methods herein also perform a second exposure using thesecond optical mask (when, for example, performing overlay measurementto determine if a photoresist is properly aligned, forming morestructures, or doing mask alignment before exposure). The secondexposure has the third Moiré target and the fourth Moiré target of bothsets of targets.

This allows the methods herein to determine interference patternmisalignment of the relatively dark and relatively light regions of thefirst interference patterns and the second interference patterns in bothpairs of conjugated interference patterns when patterns in the secondoptical mask (that are in the photoresist, or are being projected) areover the integrated circuit layer. Further, these methods calculate amagnification factor (of the interference pattern misalignment to thetarget misalignment) as a ratio of the difference of misalignment of therelatively dark and relatively light regions in the pairs ofinterference patterns, over twice the bias distance. Then, the methodsherein divide the interference pattern misalignment by the magnificationfactor to produce a self-referenced and self-calibrated targetmisalignment amount, which is then output.

Various systems herein can include, among other components, a processor;and a manufacturing system and an optical alignment measurement systemconnected to the processor over a computerized network. The processor isspecifically adapted to, or is capable of, establishing a first Moirétarget having a first pitch on a first optical mask, and establishing asecond Moiré target having a second pitch on the first optical mask. Thesecond Moiré target is adjacent to, and aligned with, the first Moirétarget. Similarly, the processor is specifically adapted to, or iscapable of, establishing a third Moiré target having the second pitch ona second optical mask, and establishing a fourth Moiré target having thefirst pitch on the second optical mask. The third Moiré target isadjacent to the fourth Moiré target. The third Moiré target ismisaligned with the fourth Moiré target by a bias distance.

The first Moiré target and the third Moiré target form firstinterference patterns. The second Moiré target and the fourth Moirétarget form second interference patterns. The first interferencepatterns and the second interference patterns form a first pair ofconjugated interference patterns.

The first Moiré target, the second Moiré target, the third Moiré target,and the fourth Moiré target are a first set of targets. The processor isspecifically adapted to, or is capable of, establishing a second set oftargets that is identical to the first set of targets, and is invertedrelative to the first set of targets, on the first optical mask and thesecond optical mask to produce a second pair of conjugated interferencepatterns that is inverted relative to the first pair of conjugatedinterference patterns.

The manufacturing system is specifically adapted to, or is capable of,performing a first exposure using the first optical mask to produce anintegrated circuit layer having the first Moiré target and the secondMoiré target of both sets of targets. Similarly, the manufacturingsystem is specifically adapted to, or is capable of, performing a secondexposure using the second optical mask. The second exposure has thethird Moiré target and the fourth Moiré target of both sets of targets.

The optical alignment measurement system is specifically adapted to, oris capable of, determining interference pattern misalignment ofrelatively dark and relatively light regions of the first interferencepatterns and the second interference patterns in both pairs ofconjugated interference patterns, when photoresist patterns formed usingthe second optical mask are over the integrated circuit layer. Also, theprocessor is specifically adapted to, or is capable of, calculating amagnification factor of the interference pattern misalignment to targetmisalignment, as a ratio of the difference of misalignment of therelatively dark and relatively light regions in the pairs ofinterference patterns, over twice the bias distance. Similarly, theprocessor is specifically adapted to, or is capable of, dividing theinterference pattern misalignment by the magnification factor to produceand output a self-referenced and self-calibrated target misalignmentamount.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments herein will be better understood from the followingdetailed description with reference to the drawings, which are notnecessarily drawn to scale and in which:

FIG. 1 is a flow diagram illustrating embodiments herein;

FIGS. 2-10 are schematic diagrams of optical masks according toembodiments herein; and

FIG. 11 is a schematic diagram of a hardware system according toembodiments herein.

DETAILED DESCRIPTION

Methods and systems herein provide enhanced contrast masks used withphotolithographic processing by producing a self-referencing,self-calibrating target design, which generates a target performanceindicator. More specifically, two pairs of alignment targets (onealigned, one misaligned by a bias distance) are formed on differentmasks to produce a first pair of conjugated interference patterns. Otherpairs of alignment targets are also formed on the masks to produce asecond pair of conjugated interference patterns that are inverted thefirst. Misalignment of the dark and light regions of the firstinterference patterns and the second interference patterns in both pairsof conjugated interference patterns is determined when patterns formedusing the masks are overlaid. A magnification factor (of theinterference pattern misalignment to the target misalignment) iscalculated as a ratio of the difference of misalignment of therelatively dark and relatively light regions in the pairs ofinterference patterns, over twice the bias distance. Then, theinterference pattern misalignment is divided by the magnification factorto produce a self-referenced and self-calibrated target misalignmentamount, which is then output.

Therefore, without having to use reference marks, and without using themuch slower scanning electro-microscopy (SEM) processing (but obtainingthe same or greater alignment precision as SEM processing), the methodsand systems herein self-reference and self-calibrate the alignment ofonly Moiré interference pattern targets to provide faster feedback thanSEM processing, but with the same accuracy as SEM processing.

As also noted previously overlay tolerances are becoming increasinglycritical as required by shrinking design rules and increasing processcomplexity. Current tech nodes require tight overlay budgets, imposing asevere challenge to current overlay metrology systems. Process effectsand optical effects, such as imaging resolution and aberrations, limitimaging overlay accuracy. Often, due to its high precision and accuracy,SEM based overlay (SEMOVL) is used to calibrate optical overlay. HoweverSEMOVL suffers from long measurement time (low throughput), and notbeing able to feedback OVL information in time.

To address these and other issues, this disclosure presents aself-referenced (SR) MoiréOVL scheme that greatly enhances the overlaydelectability by magnifying the response of signal kernel to overlayshift (by a magnification factor 1/f_(SR) of more than one order, wheref_(SR) is the self-referenced magnification factor). In this disclosure,the MoiréOVL target is designed with enhanced signal contrast forimage-based overlay (IBO), which enables design of dense kernel pitches.Also, this self-referencing design greatly improves the accuracy of themeasurement. Thus, factors (such as target asymmetry) which causetraditional IBO inaccuracy are not amplified in MoiréOVL kernels, andtherefore the calibrated MoiréOVL inaccuracy is reduced by a factor of1/f_(SR). With these systems and methods, the MoiréOVL offers apossibility of achieving SEMOVL accuracy and precision, by opticaloverlay means. Further, this self-referencing MoiréOVL can beimplemented and measured in the architecture of current image basedoverlay systems.

More specifically, this disclosure presents a self-calibrated (SC)MoiréOVL in combination with the self-referencing design, to furthercalibrate the MoiréOVL magnification factor for each mark. Theself-calibration design incorporated in this MoiréOVL enables checkingthe fidelity of the magnification factor, therefore an accurate andprecise OV can be achieved, and the calibration results can be used as akey performance indicator for Moiré OVL mark/measurement. Also, theself-calibration and self-reference can be measured at the same time toincrease the throughput. The optimization of the target design throughsimulation and the characterization of mark and measurement quality canresult from this disclosure.

Thus, this disclosure presents an overlay metrology employing Moirépatterns to magnify the kernel response to overlay misalignment, thusenhancing the kernel sensitivity and delectability. A Moiré pattern isformed and observed when the top grating and bottom grating have closebut different pitches. In general, the Moiré pitch equals the maximumdensity pitch, which is resolvable by optical means, and can becalculated using the following expression:

$p_{Moire} = {\frac{p_{1}p_{2}}{p_{1} - p_{2}}.}$

With systems and methods herein, the MoiréOVL target design accommodatescurrent IBO schematics. Here, the kernels (the dark areas between thelight areas) in the Moiré interference patterns are large enough for IBOoptical resolution, which makes the OVL kernels resolvable by currentIBO tools (Optical Resolution<p_(Moiré)). Further, the Moiré pitch ismaintained small enough to accommodate, for example, at least 5 pitchesin one target, to ensure the precision of MoiréOVL is comparable, orbetter than, conventional IBO targets (p_(Moiré)<Target size/5).However, either the top or bottom grating pitch is not opticallyresolvable by IBO tools, and this minimizes the interference between thesignal from top/bottom gratings to Moiré pitches, and this enhances theMoiré pitch contrast significantly (P_(top)(P_(bottom))<OpticalResolution).

Also, this processing balances the top and bottom grating reflectingsignal strength to further enhance the signal contrast. For example, themethods and systems herein tune top/bottom grating critical dimensions(CD) to modulate signal strength. This design guideline is adopted fortarget design and optimization, and is verified on the wafer. Thecritical dimension (CD) tuning to further improve signal contrastprecision can be further simulated considering the kernel contrastmodulation by OVL stack geometries and materials.

For advanced inspection metrology (AIM) design, the number of bars canbe placed in one mark is generally limited by kernel contrast. Ifkernels (bars) are too close to each other, the kernel contrast betweenbars and spaces in between will be degraded. However, as noted above,the methods and systems herein intentionally set one or both of the topor bottom grating pitch to not be optically resolvable by IBO tools.Thus, resulting from this top and bottom optical unresolvable pitch, thedense-kernel design presented herein takes advantage of the highcontrast signal between the optically resolvable Moiré kernel and theunresolvable background pitches. Hence, the precision of the measurementis greatly increased by the dense kernel design produced by methods andsystems herein. Compared to the size of current conventional AIMtargets, this disclosed design provides robust signal contrastprecision, while saving large amounts of real estate for metrology.

Therefore, methods herein provide enhanced contrast masks used withphotolithographic processing by producing a self-referencing,self-calibrating target design, which generates a target performanceindicator. More specifically, the methods herein use Moiré targets ofparallel marks having different pitches, which produce dark and lightMoiré interference patterns (kernels) when the Moiré targets areoverlaid.

As shown in the flowchart in FIG. 1, in item 100, the methods hereinenhance target contrast by determining the minimum optical resolution ofa photolithographic optical alignment measurement system. The methodscan then establish a first pitch of a first Moiré target, and a secondpitch of an adjacent Moiré target, in item 105 by setting at least oneof the pitches below the minimum optical resolution of the opticalalignment measurement system. However, in item 105, these methods setthe difference between the first pitch and the second pitch to generateMoiré interference patterns (kernels or dark stripes) that are above theminimum optical resolution of the optical alignment measurement system.In other words, in item 105, the pitches are set so that the opticalalignment measurement system cannot detect the lines of the individualMoiré targets (the Moiré targets are optically unresolveable), but candetect the dark lines/stripes of the kernels of the Moiré interferencepatterns. These methods further enhance target contrast by establishingthe pitches to produce some minimum number (e.g., at least 5, at least10, etc.) parallel marks in each of the Moiré targets in item 105.

These methods provide additional enhanced contrast by setting the firstpitch relative to the second pitch to balance the strength of reflectionof the first Moiré target and reflection of the second Moiré target initem 105. When balancing the strength of reflection, in item 105, thesemethods determine the strength of reflection based on sizes of features(e.g., line widths, gap widths, etc.) in the Moiré targets, and based onthe transparency, thickness, surface texture/reflectivity, geometriesetc., of the materials of the layers of the integrated circuit devicebeing produced.

With respect to producing a self-referencing, self-calibrating targetdesign, which generates a target performance indicator, in item 110methods herein establish a first location for the first Moiré target,and a second location for the second, adjacent Moiré target, on a firstoptical mask. The first Moiré target has features (e.g., parallel linesseparated by spaces) occurring at a first pitch (e.g., spacing,frequency, occurrence, etc.), and the second Moiré target has similarfeatures occurring at a second pitch, which is different from the firstpitch.

The first Moiré target in the first location is adjacent to, and alignedwith, the second Moiré target in the second location on the firstoptical mask. More specifically, the Moiré targets are parallel linesand gaps between the lines running in a first direction. The first andsecond Moiré targets are immediately adjacent one another in the firstdirection on the first optical mask. Also, while the first and secondMoiré targets have different pitches, they are aligned, meaning that thecenter of gravity (COG) of each Moiré target lies along the same line(parallel to the first direction). In other words, the very center (forexample a line or a gap) of the first Moiré target lies along the sameline (in the first direction) as the very center of the second Moirétarget.

In a similar way, in item 110 the methods herein establish a thirdlocation for a third Moiré target and a fourth location for a fourthMoiré target on a second optical mask. The third location on the secondoptical mask corresponds to (is the same as) the first location on thefirst optical mask, and the fourth location on the second optical maskcorresponds to (is the same as) the second location on the first opticalmask. This causes the first and third Moiré targets to make up a firstpair of targets, and the second and fourth Moiré targets make up asecond pair of targets that are immediately adjacent to one another,allowing kernels of such pairs to be compared optically.

The third Moiré target has features similar to that discussed aboveoccurring at the second pitch, and the fourth Moiré target has similarfeatures occurring at the first pitch. In some implementations, thefirst Moiré target and the fourth Moiré target can be identical, and thesecond Moiré target and the third Moiré target can be identical.

The third Moiré target is similarly immediately adjacent to the fourthMoiré target; however, in contrast to the first optical mask, inaddition to having different pitches, the third Moiré target is notaligned with (is offset or biased relative to) the fourth Moiré targeton the second optical mask. The distance of this bias is in a seconddirection perpendicular to the first direction, and this bias distanceallows the kernels produced by the pairs of targets to beself-referencing and self-calibrating, and allows methods herein togenerate a target performance indicator. Thus, the pairs of targets areimmediately adjacent to, and aligned, with each other (even if, withinone of the pairs, the targets themselves are intentionally misaligned).

As used herein, targets that are “immediately adjacent” are spaced apartfrom one another (in the direction parallel to the lines and gaps in theMoiré targets) a close enough distance to allow optical comparison (thatis, for example, equal to less than 50 times, less than 25 times, lessthan 10 times, etc., the width of the parallel lines (or the gaps) inthe Moiré target) and may be spaced as closely as just a few (e.g., 5)of the gaps. Thus, the pairs of Moiré targets are spaced close enough toone another on the masks to allow optical observation of whether thedark lines (kernels) produced by each pair of Moiré targets are alignedby an automated optical mask alignment system.

The first Moiré target and the third Moiré target form firstinterference patterns (Moiré patterns). The second Moiré target and thefourth Moiré target also form second interference patterns (Moirépatterns). Further, the first interference patterns and the secondinterference patterns form a first pair of conjugated interferencepatterns.

Also, the first Moiré target, the second Moiré target, the third Moirétarget, and the fourth Moiré target are a first set of targets. Suchmethods further establish a second set of targets that is identical tothe first set of targets, and is inverted relative to the first set oftargets, on the first optical mask and the second optical mask toproduce a second pair of conjugated interference patterns that isinverted relative to the first pair of conjugated interference patterns.This additional second set of identical targets includes the samefirst-fourth Moiré targets that are described above. The only differencebetween the first-fourth Moiré targets in the second set of targets, isthat such targets inverted relative to the first set of targets.

The first optical mask and the second optical mask are thus elements ofa photolithographic integrated circuit manufacturing system. In order tomanufacture integrated circuit structures, in item 115, the methodsherein obtain (manufacture or procure) the first optical mask with thefirst Moiré target and the second Moiré target (of both sets of targets)thereon, and obtain the second optical mask with the third Moiré targetand the fourth Moiré target (of both sets of targets) thereon.

Then, such methods perform a first exposure using the first optical maskin the manufacturing system, as part of the process of forming featureson a layer of the integrated circuit structure, in item 120. The firstexposure produces first markings corresponding to the first Moiré targetin a location on the layer of the integrated circuit structurecorresponding to the first location, and second markings correspondingto the second Moiré target in a location on the layer of the integratedcircuit structure corresponding to the second location.

Similarly, in item 125, these methods perform a second exposure of theintegrated circuit structure, aligned with the location of the firstexposure, using the second optical mask in the manufacturing system.This second exposure can be for additional features on the same layer onwhich the first exposure formed features (when using different colormasks), or can be for features on a second layer of the integratedcircuit structure being added on top of the first layer, such as anadditional photoresist, or additional functional layer.

Note that these methods are useful for both an overlay measurementapplication, and a scanner alignment application. Therefore, the thirdand fourth markings discussed herein can be actual markings appearing ona formed photoresist or the integrated circuit structure (wherealignment of features is checked after lines are formed in thestructure) if actual structures are formed in item 125; or, as shown initem 127, the third and fourth markings can be light projections used toalign the second optical mask with the just-formed lines on theintegrated circuit structure from the first exposure (scanneralignment). Note that, as shown by the dashed boxes and lines in FIG. 1,if the processing is used for scanner alignment in item 127, oncealignment processing is completed in item 140, the processing returns toitem 125 to form actual structures (which can then be subject to overlaymeasurement application in items 130-140).

The second exposure produces third markings corresponding to the thirdMoiré target in a location of the photoresist or the integrated circuitstructure corresponding to the third location, and fourth markingscorresponding to the fourth Moiré target in a location of thephotoresist or the integrated circuit structure corresponding to thefourth location (for each of the sets of targets).

The first markings and the third markings combine to form firstinterference patterns having dark and light portions produced by acombination of the patterns of the first Moiré target and the thirdMoiré target. Also, the second markings and the fourth markings combineto form second interference patterns having dark and light portionsproduced by a combination of the patterns of the second Moiré target andthe fourth Moiré target.

The first location and the second location on the first optical mask,and the third location and the fourth location on the second opticalmask established are positioned in item 110 to align all the dark andlight portions of the first interference patterns and the secondinterference patterns, when the first optical mask and the secondoptical mask are aligned to the same location when used for exposure.This allows these methods to determine the misalignment amount of theinterference patterns based on how closely the dark and light portionsof the first interference patterns and the second interference patterns(the kernels) are aligned in item 130, using an optical alignmentmeasurement system that has sufficient resolution to detect those darkand light portions of Moiré interference patterns. Thus, in item 130,these methods determine interference pattern misalignment of therelatively dark and relatively light regions of the first interferencepatterns and the second interference patterns in both pairs ofconjugated interference patterns when patterns in the second opticalmask are over the integrated circuit layer.

The difference in the pitches causes an expected magnification of thefirst and second interference patterns, assuming no distortion createdby the manufacturing system. However, distortion is likely present and,therefore, as shown in item 135, these methods calculate a calibratedmagnification factor (of the interference pattern misalignment to thetarget misalignment) as a ratio of the difference of misalignment of therelatively dark and relatively light regions in the pairs ofinterference patterns, over twice the bias distance.

Therefore, such processing first calculates the self-calibratedmagnification factor as a ratio of the difference of misalignment ofkernels in the pairs of interference patterns, over twice the biasdistance in item 135. Further, methods herein can track the calibrationfactor and misalignment amount for a given system to generateperformance indicators for specific targets and/or systems in item 137.In other words, in item 137, the calibration factors of variousindividual masks, sets of masks, types of masks, etc., as well asdifferent systems that produce or use masks (foundries, mask houses,etc.) can be tracked over time to rate such masks or systems, anddetermine how such masks or systems are performing. In other words, theperformance indicators generated in item 137 show how closely the masksor systems are to the expected magnification (which is free ofdistortion).

Then, this processing uses the self-calibrated magnification factor fromitem 135 to calculate actual target overlay shift (mask misalignment),which equals the COG shift between top and bottom layers, divided by theself-calibrated magnification factor. Thus, in item 140, the methodsherein divide the interference pattern misalignment by the magnificationfactor to produce a self-referenced and self-calibrated targetmisalignment amount, which is then output. Thus, in item 140, theoverlay shift of the targets on the developed resist and underlyinglayer is fed back to the scanner to provide overlay offset inline.

FIGS. 2-10 illustrate the foregoing using some exemplary Moiré targets.More specifically, FIG. 2 illustrates a portion of a first mask 200having a first location 204 for a first Moiré target 212, and a secondlocation 206 for a second Moiré target 214. FIG. 3 illustrates the firstMoiré target 212 in the first location 204, and the second Moiré target214 in the second location 206. FIG. 3 also illustrates that the firstMoiré target 212 has features (e.g., parallel lines) occurring at afirst pitch (e.g., spacing, frequency, occurrence, etc.), and the secondMoiré target 214 has similar features occurring at a second pitch, whichis different from the first pitch.

As shown in FIGS. 2 and 3, the first Moiré target 212 in the firstlocation 204 is adjacent to, and aligned with, the second Moiré target214 in the second location 206 on the first optical mask 200. Morespecifically, as shown in FIG. 3, the Moiré targets are parallel linesand gaps between the lines running in a certain (e.g., first) direction.The first and second Moiré targets 212 and 214 are immediately adjacentone another (in the first direction). Also, while the first and secondMoiré targets 212 and 214 have different pitches, they are aligned,meaning that the center of gravity (COG) of each Moiré target lies alongthe same line (parallel to the first direction). The center of gravityis the location at the very middle (midline) of a target. In otherwords, the very center of the first Moiré target 212 lies along the sameline (in the first direction) as the very center of the second Moirétarget 214.

In a similar way, as shown in FIG. 4, the methods herein establish athird location 208 for a third Moiré target 216 and a fourth location210 for a fourth Moiré target 218 on a second optical mask 202. FIG. 5illustrates the third Moiré target 216 in the third location 208, andthe fourth Moiré target 218 in the fourth location 210. FIG. 5 alsoillustrates that the third Moiré target 216 has features similar to thatdiscussed above occurring at the second pitch, and the fourth Moirétarget 218 has similar features occurring at the first pitch. In someimplementations, the first Moiré target 212 and the fourth Moiré target218 can be identical, and the second Moiré target 214 and the thirdMoiré target 216 can be identical. The first and third Moiré targets 212and 216 make up a first pair of targets, and the second and fourth Moirétargets 214 and 218 make up a second pair of targets.

The third Moiré target 216 is similarly immediately adjacent to thefourth Moiré target 218; however, in contrast to the first optical mask200, in addition to having different pitches, the third Moiré target 216is not aligned with (is offset or biased relative to) the fourth Moirétarget 218 on the second optical mask 202. In other words, the center ofgravity of the third and fourth Moiré targets 216 and 218 do not liealong the same line, as shown in FIG. 5. As shown in FIG. 5, thedistance d of this bias is in a second direction perpendicular to thefirst direction, and this bias distance allows a pair of interferencepatterns to be self-referencing and self-calibrating, and allows methodsherein to generate a target performance indicator.

As noted above, the methods herein enhance target contrast bydetermining the minimum optical resolution of a photolithographicoptical alignment measurement system. The methods can then establish thepitches of each pair of Moiré targets, by setting at least one of suchpitches below the minimum optical resolution of the optical alignmentmeasurement system. However, these methods set the difference betweenthe pitches to generate Moiré interference patterns that are above theminimum optical resolution of the optical alignment measurement system.In other words, the optical alignment measurement system cannot detectthe lines of the individual Moiré targets, but can detect the light anddark patterns that make up the kernels. These methods further enhancetarget contrast by establishing the pitches to produce at least fiveparallel marks in each of the Moiré targets.

These methods provide additional enhanced contrast by setting the firstpitch relative to the second pitch of each pair of Moiré targets tobalance the strength of reflection of the targets. When balancing thestrength of reflection, these methods determine the strength ofreflection based on sizes of features (e.g., line widths, gap widths,etc.) in the Moiré targets, and based on the transparency, thickness,surface texture/reflectivity, geometries etc., of the materials of thelayers of the integrated circuit device being produced.

Note that these methods are useful for both an overlay measurementapplication, and a scanner alignment application. Therefore, the thirdand fourth markings discussed herein can be actual markings appearing onthe integrated circuit structure (where alignment of features arechecked after they are formed in the structure); or the third and fourthmarkings can be light projections used to align the second optical mask202 with the previously formed marks in the substrate produced by thefirst exposure.

Therefore, the first location 204 and the third location 208 are thesame location on the first and second masks 200 and 202. However, thesecond location 206 and the fourth location 210 are the differentlocations on the first and second masks 200 and 202. More specifically,the second location 206 is displaced from the fourth location 210 by thedistance d.

FIG. 6 illustrates a substrate 220 that has been patterned with thefirst and second masks 200 and 202 aligned at the same location. Asshown in FIG. 6, first markings (corresponding to the first Moiré target212 in a location corresponding to the first location 204) and thirdmarkings (corresponding to the third Moiré target 216 in a locationcorresponding to the third location 208) combine to form firstinterference patterns 222 having dark and light portions (Moiréinterference patterns) produced by a combination of the patterns of thefirst Moiré target 212 and the third Moiré target 216. Also, the secondmarkings (corresponding to the second Moiré target 214 in a locationcorresponding to the second location 206) and the fourth markings(corresponding to the fourth Moiré target 218 in a locationcorresponding to the fourth location 210) combine to form secondinterference patterns 222 having dark and light portions of Moiréinterference patterns produced by a combination of the patterns of thesecond Moiré target 214 and the fourth Moiré target 218.

Note that these methods are useful for both an overlay measurementapplication, and a scanner alignment application. Therefore, the thirdand fourth markings discussed above can be actual markings appearing ona layer or photoresist of the integrated circuit structure (wherealignment of features are checked after they are formed in a layer); orthe third and fourth markings can be light projections used to align thesecond optical mask 202 with the previously formed first and secondmarkings.

Items 226 in FIG. 6 are the “kernels” which are the dark portions of theinterference patterns. Therefore, the first location 204 and the secondlocation 206 on the first optical mask 200, and the third location 208and the fourth location 210 on the second optical mask 202 arepositioned to cause all the dark and light patterns produced by thefirst interference patterns and the second interference patterns toalign, when the first optical mask 200 and the second optical mask 202are aligned.

However, as shown by the dashed boxes 228 surrounding opposing kernels226 in the first and second interference patterns 222 and 224 in FIG. 6,the kernels 226 are not aligned (dashed boxes 228, representingpositions of the kernels 226 in FIG. 6, are not aligned). This allowsthese methods to determine the misalignment amount of the interferencepatterns based on how closely the dark and light kernels 226 of theinterference patterns 222, 224 are aligned, using an optical alignmentmeasurement system that has sufficient resolution to detect those darkand light kernels 226. Further, these methods calculate a magnificationfactor of the interference patterns 222, 224, and then divide theinterference pattern misalignment by the magnification factor to producea target misalignment amount.

As noted above, the first location 204 and the third location 208 arethe same location on the first and second masks 200 and 202. However,the second location 206 is displaced from the fourth location 210 by thedistance d. This arrangement of the targets causes the overlay kernel toshift in the opposite direction to the overlay misalignment, causing themagnification factor to double, because each difference between darkregions of kernels of the pair of interference patterns is twice themagnification factor of one of the interference patterns alone. In otherwords, because the dark regions of adjacent interference patterns movein opposite directions when there is mask misalignment, the dark regionsare twice as far apart as each is individually from a non-movingreference point, which doubles the magnification factor when compared tomeasuring the distance of one interference pattern relative to thenon-moving reference point.

This is shown, for example, in FIG. 7 where the “1^(st) kernel recovery”shows a location where lines of the first Moiré target 212 and the thirdMoiré target 216 align, which is not the correct location if the firstand second masks 200 and 202 had been properly aligned. In order todetermine alignment, methods and systems herein calculate amagnification factor of the interference patterns, and then divide theinterference pattern misalignment by the magnification factor to producea target misalignment amount. FIG. 7 is used to demonstrate aspects ofcalculation of the magnification factor.

The expanded portion within FIG. 7 illustrates the width of the lines(d₂) and the spacing or pitch (p₂) the first Moiré target 212; as wellas the width of the lines (d₁) and the spacing or pitch (p₁) the thirdMoiré target 216. Further, the kernel shift of the first and third Moirétargets 212 and 216 is shown in FIG. 7 as x_(o), x_(o1), etc.

For calculation of the magnification factor, with reference to FIG. 7,in the case of p₁>p₂ the kernel magnification factor (f) is set at the1^(st) pitch, x_(o1)=(p₂+x₀−p₁); Δx₀=x₀−x₀₁=p₁−p₂. Therefore, to recoverto the 0 kernel offset, the number of p₁ grating shift equalsx₀/Δx₀=x₀/(p₁−p₂). The maximum Moiré density shifts, for x₀*p₁/(p₁−p₂),to the x₀ direction.

Thus, the overlay kernel shift OVL (x₀) is calculated by OVL x ₀ =x ₀ *p₁/(p ₁ −p ₂).  (1)

This correlates OVL and Moiré pitch p _(Moiré) by OVL (x ₀)=x ₀ *p_(Moiré) /p ₂.  (2)

Therefore, this disclosure defines the magnification factor to be f=p₁/(p ₁ −p ₂).  (3)

In one example, p₁ can be 200 and p₂ can be 190. Using formula (3), thisbecomes f=200/(200−190)=20. Therefore, in this example the expectedmagnification factor is 20, meaning that misalignment of the kernels inthe interference patterns will be 20 times as great as the maskmisalignment, without distortion. This allows the expected maskmisalignment factor to be based on the design of the targets (thepitches of the targets) and not on any measured observation. However,the magnification factor is calibrated to the observed misalignment, asdiscussed below.

In the conjugated case with p₁<p₂, the magnification factor formula (3)is still valid, and results in a negative sign. This causes the overlaykernel to shift in the opposite direction to the overlay misalignment,causing the magnification factor to double, because each differencebetween dark regions of kernels of the pair of interference patterns istwice the magnification factor of one of the interference patternsalone. In other words, because the dark regions of adjacent interferencepatterns move in opposite directions when there is mask misalignment,the dark regions are twice as far apart as each is individually from anon-moving reference point, which doubles the magnification factor whencompared to measuring the distance of one interference pattern relativeto the non-moving reference point.

Therefore, the self-referencing MoiréOVL takes advantage of the kernelmagnification effect and the reverse sign effect for conjugated pitchconfigurations (p₁>p₂, p₁<p₂) to avoid the use of reference points ormarks, and to double the magnification factor. Therefore, the methodsand systems herein are employed to overlay kernels for self-referenceand, the self-reference doubles the magnification factor compared with asingle Moiré pattern aligned to a non-moving reference point.

More specifically, for conjugated pitch settings (p_(a), p_(b)) in whichp_(a)>p_(b), the target design is Target1: p₁=p_(a); p₂=p_(b) andTarget2: p₁=p_(a); p₂=p_(b). The kernel magnification factor for thesetargets is therefore:

$\begin{matrix}{{{Target}\; 1\text{:}\mspace{14mu} f_{1}} = {\frac{p_{1}}{p_{1} - p_{2}} = \frac{p_{a}}{p_{a} - p_{b}}}} & (4) \\{{{Target}\; 2\text{:}\mspace{14mu} f_{2}} = {\frac{p_{1}}{p_{1} - p_{2}} = \frac{p_{a}}{p_{a} - p_{b}}}} & (4)\end{matrix}$

The self-referenced kernel shift is shown by

OVL(x ₀)_(SA)=OVL(x ₀)₁−OVL(x ₀)₂  (5)

Together with (1), (4), and (5)

$\begin{matrix}{{{OVL}\left( x_{0} \right)}_{SA} = {\left( \frac{p_{a} + p_{b}}{p_{a} - p_{b}} \right) = \left( x_{0} \right)}} & (6)\end{matrix}$

Therefore, the self-referenced kernel magnification factor (f_(SR)) canbe represented as:

$\begin{matrix}{f_{SR} = \frac{p_{a} + p_{b}}{{p_{a} - p_{b}}}} & (7)\end{matrix}$

The first Moiré target and the third Moiré target form firstinterference patterns. The second Moiré target and the fourth Moirétarget also form second interference patterns. Further, the firstinterference patterns and the second interference patterns form a firstpair of conjugated interference patterns.

The first Moiré target, the second Moiré target, the third Moiré target,and the fourth Moiré target are a first set of targets. Such methodsfurther establish a second set of targets that is identical to the firstset of targets, and that is inverted relative to the first set oftargets, on the first optical mask and the second optical mask, toproduce a second pair of conjugated interference patterns that isinverted relative to the first pair of conjugated interference patterns.

FIGS. 8 and 9 illustrate such structure, and these drawings show thatthe first and second masks 200 and 202 can include additional sets offirst-fourth Moiré targets 212, 214, 216, 218 in different orientations(e.g. inverted, perpendicular, inverted and perpendicular, etc., tothose shown in FIGS. 3 and 5) in addition to the first set of Moirétargets discussed above. Specifically FIGS. 8 and 9 illustrate four setsof four targets on the two masks, two set are perpendicular to the othertwo, and each set that is parallel is inverted with respect to theother.

FIG. 10 illustrates the various kernels that can be produced from theoverlay of the marks resulting from the masks shown in FIGS. 8 and 9.With respect to magnification calibration, in this example there is oneset of four targets (or kernel pair (kernels A and B)) and acorresponding identical, but inverted second set of four targets (orkernel pair (kernels A′ and B′)) in FIG. 10. More specifically, in thisexample, if kernels A and B have bias d, then kernels A and B have bias−d because kernels A′ and B′ are inverted relative to kernels A and B.Therefore, with misalignment amount (actual OV shift) referred to as OV,the kernel shift between A and B is A−B=(d+OV)*f_(SC). Similarly, thekernel shift between A′ and B′ is A′−B′=(−d+OV)*f_(SC). Therefore, theSC magnification factor is calculated by the kernel shift offset between(A−B) and (A′−B′) which is:

$f_{SC} = \frac{\left( {A - B} \right) - \left( {A^{\prime} - B^{\prime}} \right)}{2d}$

and the actual OV can be calculated as:

${OV} = {\frac{{\left( {A + A^{\prime}} \right)/2} - {\left( {B + B^{\prime}} \right)/2}}{f_{SC}}.}$

In other words, the expected magnification factor is calculated from thedesign of the targets, and is based on the difference between thedesigned pitches of the overlaid targets. However, the actualmagnification factor will be different because of distortions caused bythe photolithographic system. Rather than using a time consuming SEM tomeasure the actual magnification, the methods and systems hereincalculate the actual magnification based on the differences of therelatively inverted pairs of interference patterns (one pair: kernels Aand B; second pair kernels A′ and B′).

As shown by the above calculations, this processing first finds theself-calibrated magnification factor (f_(SC)) as a ratio of thedifference of misalignment of kernels in the pairs of interferencepatterns ((A−B) less (A′−B′)) over twice the bias distance (2 d). Thenthis processing uses the self-calibrated magnification factor (f_(SC))to calculate actual OV shift (actual mask misalignment), which as shownabove, equals the averaged COG shift between top and bottom layers((A+A′)/2−(B+B′)/2) divided by the self-calibrated magnification factor(f_(SC)). Self-calibration (SC) and self-referencing (SR) processes aretherefore integrated and are calculated in one operation, withoutsacrificing target space or measurement throughput.

Because of the pitch differences of the first and third targets 212 and216, and the pitch differences of the second and fourth targets 214 and218, the interference patterns are calculated to produce an expectedmagnification of the alignment of the lines in the targets. Therefore,in addition to being self-referencing, the methods and systems hereinare also self-calibrating. The calibration integrated in SR-MoiréOVLcalibrates the magnification factor for individual marks on thewafer/mask. This processing detects the deviation of the magnificationfactor from the design's theoretical (undistorted/unmagnified) value.This deviation maybe caused by mark damage, etc. The self-calibratedmagnification factor (f_(SC)) is fed back to the measurement results tocalibrate the actual OVL shift using Actual OVL shift=MoiréOVL/f_(SC).

How far the calibrated factor deviates from the theoretical value is akey performance indicator (KPI) for mark quality and measurement qualityas follows

${{Moiré}\; {OVL}\text{-}{KPI}} = \frac{{f_{SR} - {f_{SC}}}}{f_{SR}}$

Therefore, such processing determine interference pattern misalignmentof the relatively dark and relatively light regions of the firstinterference patterns and the second interference patterns in both pairsof conjugated interference patterns when patterns in the second opticalmask are over the integrated circuit layer. Thus, a magnification factor(of the interference pattern misalignment to the target misalignment) iscalculated as a ratio of the difference of misalignment of therelatively dark and relatively light regions in the pairs ofinterference patterns, over twice the bias distance. Then, theprocessing herein divides the interference pattern misalignment by themagnification factor to produce a self-referenced and self-calibratedtarget misalignment amount, which is then output.

Because the methods and systems herein utilize lines in the Moirétargets that are too small to be recognized by optical systems; butprovide a self-referenced, self-calibrated misalignment amount usingoptical systems that can recognize the Moiré interference patterns, themethods and systems herein can eliminate the need to perform alignmentusing slower throughput systems such as scanning electron microscopes.Therefore, the methods and systems herein can eliminate the necessity ofusing SEMOVL for the OVL final calibration. This is especially truebecause the SR-Moiré OVL disclosed above offers comparable accuracy andprecision as SEMOVL, which permits removing the SEMOVL in production asthe OVL final calibration.

In addition, the methods and systems herein reduce OVL inaccuracy,because OV inaccuracy factors (such as grating asymmetry) are notamplified in MoiréOVL, and such are reduced by a factor of1/magnification with methods and systems herein. Additionally, withmethods and systems herein, the self-calibration offers a good KPI formark/measurement quality, without sacrificing throughput. The integratedself-calibration processing herein calculates the magnification factorfor each mark on the wafer individually. The deviation from the designmagnification factor is used as a key performance indicator of the markquality. Further, the methods and systems provide shrinkage of targetsize. The high accuracy and precision of the Moiré target provided bymethods and systems herein allow such Moiré targets to be reduced insize.

In addition, the methods and systems herein minimize the requirementsfor imaging tool. In the MoiréOVL processing described above, the kernelCD, pitches and kernel magnification factor can be adjusted by changingtop and bottom gratings. Image resolution is not required to captureoverlay misalignment directly, and by adjusting the magnificationfactor, nanometer scale overlay offset can be magnified larger than oneorder. Magnification effects greatly lower the requirement for opticabbreviations and imaging resolution. Therefore, currentstate-of-the-art IBO systems can fully support the previously describedMoiréOVL processing. This allows current AIM overlay to be easilyimplemented in the SR-MoiréOVL process described above.

Also, the methods and systems herein are more stable to processvariations. With the MoiréOVL process presented above, the kernel CD,and pitches are defined by pattern density variations. Process variationchanges the contrast of gratings, however pattern density of Moirépitches are less affected by the contrast, allowing more stability.

Various systems herein can include, among other components, a processor320; and a manufacturing system 310 and an optical alignment measurementsystem 314 connected to the processor 320 over the computerized network322. The processor 320 is specifically adapted to, or is capable of,establishing a first location for a first Moiré target on a firstoptical mask, such that the first Moiré target has features occurring ata first pitch. The processor 320 is also specifically adapted to, or iscapable of, establishing a second location for a second Moiré target onthe first optical mask, such that the second Moiré target has featuresoccurring at a second pitch, different from the first pitch, and suchthat the first Moiré target is adjacent to, and aligned with, the secondMoiré target on the first optical mask.

The processor 320 is similarly specifically adapted to, or is capableof, establishing a third location for a third Moiré target on a secondoptical mask, such that the third Moiré target has features occurring atthe second pitch. The processor 320 is further specifically adapted to,or is capable of, establishing a fourth location for a fourth Moirétarget on the second optical mask, such that the fourth Moiré target hasfeatures occurring at the first pitch, and such that the third Moirétarget is adjacent to, and not aligned with, the fourth Moiré target onthe second optical mask.

The first interference patterns and the second interference patternsform a first pair of conjugated interference patterns. The processor 320is specifically adapted to, or is capable of, establishing additionalidentical targets on the first optical mask and the second optical maskto produce a second pair of conjugated interference patterns that isinverted relative to the first pair of conjugated interference patterns.

Moving to the manufacturing system 310, a mask house 300 (or similar)with a mask production unit 302 is included in such a system forpurposes herein, and is specifically adapted to, or is capable of,producing the first optical mask with the first Moiré target and thesecond Moiré target thereon. The manufacturing system 310 is similarlyspecifically adapted to, or is capable of, producing the second opticalmask with the third Moiré target and the fourth Moiré target thereon.

Additionally, the manufacturing system 310 includes a fabricationfacility (wafer fab, or similar) having a photolithographic exposureunit 312 for purposes herein that is specifically adapted to, or iscapable of, exposing a first exposure of an integrated circuit structureusing the first optical mask in the manufacturing system 310, such thatthe first exposure produces first markings corresponding to the firstMoiré target in a location corresponding to the first location, andsecond markings corresponding to the second Moiré target in a locationcorresponding to the second location. The manufacturing system 310 issimilarly specifically adapted to, or is capable of, exposing/forming asecond exposure of the integrated circuit structure aligned with thelocation of the first exposure using the second optical mask in themanufacturing system 310, such that the second exposure produces thirdmarkings corresponding to the third Moiré target in a locationcorresponding to the third location, and fourth markings correspondingto the fourth Moiré target in a location corresponding to the fourthlocation.

As noted above, the first markings and the third markings combine toform first interference patterns having dark and light portions of Moiréinterference patterns produced by a combination of the patterns of thefirst Moiré target and the third Moiré target, and wherein the secondmarkings and the fourth markings combine to form second interferencepatterns having dark and light portions of Moiré interference patternsproduced by a combination of the patterns of the second Moiré target andthe fourth Moiré target.

The processor 320 is further specifically adapted to, or is capable of,establishing the first pitch and the second pitch by setting the firstpitch and the second pitch below the minimum optical resolution, settinga difference between the first pitch and the second pitch to generatethe first interference patterns and the second interference patternsthat are above the minimum optical resolution, and setting the firstpitch relative to the second pitch to balance the strength of reflectionof the first Moiré target and reflection of the second Moiré target.More specifically, the processor 320 establishes the first pitch and thesecond pitch by setting the first pitch and the second pitch to produceat least five parallel marks in each of the first Moiré target, thesecond Moiré target, the third Moiré target, the fourth Moiré target.Additionally, the processor 320 sets the first pitch relative to thesecond pitch to balance the strength of reflection by determining thestrength of reflection based on the sizes of features in the first Moirétarget, the second Moiré target, the third Moiré target, the fourthMoiré target; and based on the transparency characteristics of materialsand geometries of the first exposure and the second exposure.

The first location and the second location on the first optical mask,and the third location and the fourth location on the second opticalmask, are positioned to align all the dark and light portions of thefirst interference patterns and the second interference patterns, whenthe first optical mask and the second optical mask are aligned when usedin the manufacturing system 310.

The optical alignment measurement system 314 is specifically adapted to,or is capable of, determining interference pattern misalignment of therelatively dark and relatively light regions of the first interferencepatterns and the second interference patterns in both pairs ofconjugated interference patterns, when patterns in the second opticalmask are over the integrated circuit layer. Also, the processor 320 isspecifically adapted to, or is capable of, calculating a magnificationfactor of the interference pattern misalignment to target misalignment,as a ratio of the difference of misalignment of the relatively dark andrelatively light regions in the pairs of interference patterns, overtwice the bias distance. Similarly, the processor 320 is specificallyadapted to, or is capable of, dividing the interference patternmisalignment by the magnification factor to produce and output aself-referenced and self-calibrated target misalignment amount.

When patterning any material herein, the material to be patterned can begrown or deposited in any known manner and a patterning layer (such asan organic photoresist) can be formed over the material. The patterninglayer (resist) can be exposed to some pattern of light radiation (e.g.,patterned exposure, laser exposure, etc.) provided in a light exposurepattern, and then the resist is developed using a chemical agent. Thisprocess changes the physical characteristics of the portion of theresist that was exposed to the light. Then one portion of the resist canbe rinsed off, leaving the other portion of the resist to protect thematerial to be patterned (which portion of the resist that is rinsed offdepends upon whether the resist is a negative resist (illuminatedportions remain) or positive resist (illuminated portions are rinsedoff). A material removal process is then performed (e.g., wet etching,anisotropic etching (orientation dependent etching), plasma etching(reactive ion etching (RIE), etc.)) to remove the unprotected portionsof the material below the resist to be patterned. The resist issubsequently removed to leave the underlying material patternedaccording to the light exposure pattern (or a negative image thereof).

While only one or a limited number of masks are illustrated in thedrawings, those ordinarily skilled in the art would understand that manydifferent types mask could be simultaneously formed with the embodimentherein and the drawings are intended to show simultaneous formation ofmultiple different types of masks; however, the drawings have beensimplified to only show a limited number of masks for clarity and toallow the reader to more easily recognize the different featuresillustrated. This is not intended to limit this disclosure because, aswould be understood by those ordinarily skilled in the art, thisdisclosure is applicable to structures that include many of each type ofmask shown in the drawings.

The flowchart and block diagrams in the figures illustrate thearchitecture, functionality, and operation of possible implementationsof devices and methods according to various embodiments. In this regard,each block in the flowchart or block diagrams may represent a module,segment, or portion of instructions, which includes one or moreexecutable instructions for implementing the specified logicalfunction(s). In some alternative implementations, the functions noted inthe block may occur out of the order noted in the figures. For example,two blocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts or carry outcombinations of special purpose hardware and computer instructions.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the foregoing. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. Furthermore, as used herein, terms such as “right”, “left”,“vertical”, “horizontal”, “top”, “bottom”, “upper”, “lower”, “under”,“below”, “underlying”, “over”, “overlying”, “parallel”, “perpendicular”,etc., are intended to describe relative locations as they are orientedand illustrated in the drawings (unless otherwise indicated) and termssuch as “touching”, “in direct contact”, “abutting”, “directly adjacentto”, “immediately adjacent to”, etc., are intended to indicate that atleast one element physically contacts another element (without otherelements separating the described elements). The term “laterally” isused herein to describe the relative locations of elements and, moreparticularly, to indicate that an element is positioned to the side ofanother element as opposed to above or below the other element, as thoseelements are oriented and illustrated in the drawings. For example, anelement that is positioned laterally adjacent to another element will bebeside the other element, an element that is positioned laterallyimmediately adjacent to another element will be directly beside theother element, and an element that laterally surrounds another elementwill be adjacent to and border the outer sidewalls of the other element.

Embodiments herein may be used in a variety of electronic applications,including but not limited to advanced sensors, memory/data storage,semiconductors, microprocessors and other applications. A resultingdevice and structure, such as an integrated circuit (IC) chip can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present embodiments has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the embodiments in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the embodiments herein.The embodiments were chosen and described in order to best explain theprinciples of such, and the practical application, and to enable othersof ordinary skill in the art to understand the various embodiments withvarious modifications as are suited to the particular use contemplated.

While the foregoing has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe embodiments herein are not limited to such disclosure. Rather, theelements herein can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope herein.Additionally, while various embodiments have been described, it is to beunderstood that aspects herein may be included by only some of thedescribed embodiments. Accordingly, the claims below are not to be seenas limited by the foregoing description. A reference to an element inthe singular is not intended to mean “one and only one” unlessspecifically stated, but rather “one or more.” All structural andfunctional equivalents to the elements of the various embodimentsdescribed throughout this disclosure that are known or later, come to beknown, to those of ordinary skill in the art are expressly incorporatedherein by reference and intended to be encompassed by this disclosure.It is therefore to be understood that changes may be made in theparticular embodiments disclosed which are within the scope of theforegoing as outlined by the appended claims.

What is claimed is:
 1. A structure comprising: a substrate including afirst layer and a second layer overlaying the first layer; a firstfeature having a first pitch, wherein the first feature is in a firstlocation of the first layer; a second feature having a second pitchdifferent than the first pitch, wherein the second feature is in asecond location of the first layer that is adjacent to the firstlocation; a third feature having the second pitch, wherein the thirdfeature is in a third location of the second layer that corresponds tothe first location of the first layer; and a fourth feature having thefirst pitch, wherein the fourth feature is in a fourth location of thesecond layer that corresponds to the second location of the first layer,wherein the first feature is aligned with the second feature, and thethird feature is misaligned with the fourth feature.
 2. The structureaccording to claim 1, wherein the first feature and the third featureare structured to form first interference patterns, and the secondfeature and the fourth feature are structured to form secondinterference patterns.
 3. The structure according to claim 2, whereinthe first pitch and the second pitch are below a minimum opticalresolution of an optical alignment measurement system, and wherein thefirst interference patterns and the second interference patterns areabove the minimum optical resolution.
 4. The structure according toclaim 1, wherein each of the first feature, the second feature, thethird feature and the fourth feature comprise at least five Moiréparallel line features.
 5. The structure according to claim 1, whereinthe second location is aligned to the first location, and the fourthlocation is misaligned with the third location by a bias distance. 6.The structure according to claim 1, wherein the first pitch is greaterthan the second pitch, or the first pitch is less than the second pitch.7. A method comprising: establishing a first target having a first pitchon a first mask; establishing a second target having a second pitch onthe first mask, wherein the second target is adjacent to the firsttarget; establishing a third target having the second pitch on a secondmask; and establishing a fourth target having the first pitch on thesecond mask, wherein the third target is adjacent to the fourth target,wherein the first target is aligned with the second target, wherein thethird target is misaligned with the fourth target, wherein the firsttarget and the third target form first interference patterns, andwherein the second target and the fourth target form second interferencepatterns.
 8. The method according to claim 7, further comprising:determining a minimum optical resolution of an optical alignmentmeasurement system; and establishing the first pitch and the secondpitch by: setting the first pitch and the second pitch below the minimumoptical resolution; setting a difference between the first pitch and thesecond pitch to generate the first interference patterns and the secondinterference patterns that are above the minimum optical resolution; andsetting the first pitch relative to the second pitch to balance astrength of reflection of the first target and reflection of the secondtarget.
 9. The method according to claim 8, wherein the establishing thefirst pitch and the second pitch further comprises setting the firstpitch and the second pitch to produce at least five parallel marks ineach of the first target, the second target, the third target, thefourth target.
 10. The method according to claim 8, wherein the settingthe first pitch relative to the second pitch to balance a strength ofreflection comprises determining the strength of reflection based on:sizes of features in the first target, the second target, the thirdtarget, the fourth target; and transparency characteristics of materialsand geometries of layers being manufactured.
 11. The method according toclaim 7, further comprising: performing a first exposure using the firstoptical mask to produce the first layer having the first target and thesecond target of the first set of targets and the second set of targets;and performing a second exposure using the second optical mask, whereinthe second exposure projects the third target and the fourth target ofthe first set of targets and the second set of targets on the firstlayer.
 12. The method according to claim 7, further comprising:determining interference pattern misalignment of the first interferencepatterns and the second interference patterns; calculating amagnification factor of the first interference patterns and the secondinterference patterns; and dividing the interference patternmisalignment by the magnification factor to produce and output an actualmisalignment amount.
 13. The method according to claim 12, wherein adifference between the first pitch and the second pitch causes anexpected magnification of the first interference patterns and the secondinterference patterns based on there being no distortion created by amanufacturing system, and wherein the magnification factor is differentfrom the expected magnification by a calibration factor that is based onoptically measured differences of the interference pattern misalignmentwhich makes the actual misalignment amount self-calibrating.
 14. Amethod comprising: establishing a first target having a first pitch on afirst layer using a first mask; establishing a second target having asecond pitch on the first layer using the first mask, wherein the secondtarget is adjacent to the first target; projecting a third target havingthe second pitch on the first layer using a second mask; and projectinga fourth target having the first pitch on the first layer using thesecond mask, wherein the third target is projected adjacent to thefourth target, wherein the first target is aligned with the secondtarget, wherein the third target is misaligned with the fourth target,wherein the first target and the third target form first interferencepatterns, and wherein the second target and the fourth target formsecond interference patterns.
 15. The method according to claim 14,further comprising: determining a minimum optical resolution of anoptical alignment measurement system; and establishing the first pitchand the second pitch by: setting the first pitch and the second pitchbelow the minimum optical resolution; setting a difference between thefirst pitch and the second pitch to generate the first interferencepatterns and the second interference patterns that are above the minimumoptical resolution; and setting the first pitch relative to the secondpitch to balance a strength of reflection of the first target andreflection of the second target.
 16. The method according to claim 15,wherein the establishing the first pitch and the second pitch furthercomprises setting the first pitch and the second pitch to produce atleast five parallel marks in each of the first target, the secondtarget, the third target, the fourth target.
 17. The method according toclaim 15, wherein the setting the first pitch relative to the secondpitch to balance a strength of reflection comprises determining thestrength of reflection based on: sizes of features in the first target,the second target, the third target, the fourth target; and transparencycharacteristics of materials and geometries of layers beingmanufactured.
 18. The method according to claim 14, further comprising:determining interference pattern misalignment of the first interferencepatterns and the second interference patterns; calculating amagnification factor of the first interference patterns and the secondinterference patterns; and dividing the interference patternmisalignment by the magnification factor to produce and output an actualmisalignment amount.
 19. The method according to claim 18, wherein adifference between the first pitch and the second pitch causes anexpected magnification of the first interference patterns and the secondinterference patterns based on there being no distortion created by amanufacturing system, and wherein the magnification factor is differentfrom the expected magnification by a calibration factor that is based onoptically measured differences of the interference pattern misalignmentwhich makes the actual misalignment amount self-calibrating.
 20. Themethod according to claim 14, further comprising: performing a firstexposure using the first optical mask to produce the first layer havingthe first target and the second target of the first set of targets andthe second set of targets; and performing a second exposure using thesecond optical mask, wherein the second exposure projects the thirdtarget and the fourth target of the first set of targets and the secondset of targets on the first layer.